Lithium ion secondary batteries have been widely employed as power supplies for small devices because they have a small volume and a high mass capacity density and is capable of taking out a high voltage. For example, lithium ion secondary batteries have been used as power supplies for mobile devices such as cellular phones and notebook personal computers. In addition to the use in small mobile devices, in recent years, lithium ion secondary batteries have been expected to be applied to large-size secondary batteries in the fields where a large capacity and a long battery life are required such as electric vehicles (EVs) and electric power storage due to consideration for environmental issues and increasing awareness for energy saving.
In presently commercially available lithium ion secondary batteries, positive electrode active materials based on LiMO2 with a layer structure (M is at least one of Co, Ni, and Mn) or LiMn2O4 with a spinel structure are generally used. Lithium ion secondary batteries having the positive electrode active material described above primarily use a charge/discharge region of 4.2 V or lower (hereinbelow, a positive electrode with an operating potential of 4.2 V or lower versus lithium metal will also be referred to as a “4 V-class positive electrode”). For negative electrode active materials, carbon materials such as graphite are used.
Meanwhile, materials in which a part of Mn of LiMn2O4 is replaced with Ni or the like are known to have a high charge/discharge region of 4.5 to 4.8 V versus lithium metal. Specifically, in spinel compounds such as LiNi0.5Mn1.5O4, Mn is present in the state of Mn4+, and oxidoreduction between Ni2+ and Ni4+ is used instead of the conventional oxidoreduction between Mn3+ and Mn4+, and therefore, a high operating voltage of 4.5 V or higher can be achieved. Such materials are referred to as “5 V-class active materials”, and have been expected to be a promising positive electrode material because the energy density can be improved by the increased voltages. Hereinbelow, the positive electrode comprising a positive electrode active material that exhibits a high operating voltage of 4.5 V or higher versus lithium metal (which may also be referred to as a “5 V-class active material” or a “5 V-class positive electrode active material) may also be referred to as a “5 V-class positive electrode”.
However, when the potential of a positive electrode becomes higher, there arises problems such as generation of gas due to oxidative degradation of the electrolyte solution, generation of byproducts due to decomposition of the electrolyte solution, elution of metal ions such as Mn and Ni from the positive electrode active material and their precipitation on a negative electrode, which leads to degradation of the battery capacity and generation of a large amount of gas associated with charge/discharge cycles at a temperature of 40° C. or higher, and these problems have been the obstacles to practical applications of the materials.
As a method of suppressing oxidative degradation of the electrolyte solution in the positive electrode at high potentials, use of a solvent with a high oxidation resistance can be employed. For example, Patent Literature 1 describes that an electrolyte solution which comprises a phosphate ester containing fluorine as halogen (hereinafter referred to as a “fluorinated phosphate ester”) at 7 to 20% of the electrolyte solvent and further comprises chain esters and cyclic esters is a solvent having a high voltage resistance and is excellent in self-extinguishability. In Patent Literature 2, it is described that an electrolyte solution which comprises a fluorinated phosphate ester having a structure of a molecular chain terminal group of CF2H—, a cyclic carbonate with a carbon-carbon π bond, and a cyclic compound with an S═O bond has a high discharge capacity and is a solvent excellent in flame-resistant.
Generally, since it is assumed that a fluorinated solvent containing fluorine having a high electronegativity has a high voltage resistance, a solvent like this is expected to be suitable as a solvent for an electrolyte solution for the case where a 5 V-class positive electrode is used. Other examples of fluorinated solvents that can be used for lithium ion secondary batteries include the fluorinated ethers described in Patent Literatures 3 to 5. Patent Literature 6 describes a lithium secondary battery that comprises a positive electrode comprising a positive electrode active material operating at a potential of 4.5 V or higher versus lithium and a non-aqueous electrolyte solvent comprising a fluorine-containing phosphate ester.
However, since fluorinated solvents generally have a low dielectric constant and a low dissociation property of lithium salts, low compatibility with other solvents, and may have high viscosity in some cases, the ionic conductance becomes lower as compared to carbonate-based solvents that are usually used for lithium ion batteries. The fluorinated phosphate esters described above as a fluorinated solvent have higher dielectric constant and thus have a higher lithium ion dissociation property as compared to fluorinated ethers; however, the viscosity is high. On the other hand, although fluorinated ethers have a low viscosity, the dissociation property of lithium salts is low due to their low dielectric constant.
In order to inhibit generation of gas in 5 V-class positive electrodes at high temperatures, the concentration of a fluorinated solvent in the electrolyte solution is desirably as high as possible. However, when the concentration of the fluorinated solvent in the electrolyte solution is increased as described above, the ionic conductance of the electrolyte solution decreases, and thus, although it does become easier to achieve excellent charge-discharge characteristics at high temperatures, there has been a problem such that the charge-discharge characteristics at room temperature degrade and as the result, degradation of cycle characteristics at room temperature is caused. In addition, the fluorinated solvents with a high oxidation resistance may sometimes have a low resistance to reduction, and are decomposed by reduction on the negative electrode to form a film with a high resistance, which is considered as a cause of the degradation of cycle characteristics at room temperature. In other words, in lithium ion batteries that use a 5 V-class positive electrode, it has been a major problem to be solved to improve the cycle characteristics by inhibiting generation of gas in cycles at high temperatures and achieve excellent cycle characteristics at room temperature at the same time. However, in Patent Literatures 1 to 6, no specific description has been made as to these problems in lithium ion secondary batteries that use a 5 V-class positive electrode, and neither description nor suggestion of means for solving such problems has been given.